The present invention relates to a coupling arrangement for a stripline network. The arrangement is primarily intended for application in microwave antennas, but can also be used in other microwave applications.
Stripline technology is commonly used in the microwave range. A common stripline arrangement comprises two ground planes parallel to one another, and a conductor located between the ground planes and parallel to them. Dielectric material is arranged on both sides of the conductor.
A common application of stripline arrangements is to use them as power distribution networks and transmission lines. In some applications, it is necessary to couple together stripline arrangements which lie in different layers in multilayer structures, or for the stripline arrangement to be coupled to other types of transmission lines or antenna elements which are located above or below the ground plane of the stripline arrangement.
A known technique for effecting coupling between stripline arrangements in different layers is to electrically connect the conductors in the two arrangements by means of pins or plated via-holes. A shortcoming of this technique is that it requires great manufacturing precision and increases the weight of the arrangement, and that, as a result of the inevitable discontinuities, undesirable electromagnetic modes arise, which in turn gives rise to a requirement for separate arrangements for mode suppression.
Another way of effecting coupling between a stripline arrangement and another structure in another layer is to arrange a slot or another type of aperture in the intermediate ground plane. In order to achieve good coupling between the stripline conductor and the slot, some form of asymmetry is required in the stripline arrangement.
An example of such asymmetry is described in U.S. Pat. No. 5,532,643, which describes a stripline arrangement for coupling between the stripline conductor and a slot in one ground plane of the arrangement. The arrangement comprises plates made of a first dielectric material on one side of the stripline conductor, and plates made of a second dielectric material on the other side of the stripline conductor, the first and the second dielectric materials having different dielectric constants. This arrangement gives rise to the asymmetry which is required in order for the conductor to couple to the slot and also to the conductor in the next stripline arrangement.
A shortcoming of this arrangement is that, since it requires different dielectric materials on both sides of the stripline conductor, air cannot be used throughout as the dielectric material in the arrangement. This is a disadvantage because most dielectric materials other than air result in high losses at high frequencies, which makes the arrangement difficult to use within that part of the microwave range. Furthermore, the majority of dielectric materials with good properties at high frequencies are expensive, which increases the cost of the arrangement.
The present invention provides an arrangement for coupling between apertures in the ground planes in a stripline network, which suppresses undesirable modes and provides a good degree of coupling. For the suppression of undesirable modes, the arrangement is free from discontinuities, and in order to achieve good coupling, the arrangement exhibits electrical asymmetry.
The coupling arrangement comprises a first and a second ground plane arranged essentially parallel to one another and extending in a common main direction and each having at least one aperture. A stripline conductor is arranged between the first and the second ground plane. A second dielectric layer is located between the stripline conductor and the second ground plane.
The stripline conductor includes a first main surface facing towards the first ground plane, and a second main surface towards the second ground plane. In connection to the apertures of the ground planes, the distance from the aperture to the most closely located main surface of the stripline conductor is considerably exceeded by the distance from the main surface to the other ground plane.
This relationship between the distances from each respective aperture to the other ground plane and to the stripline conductor means provides electrical asymmetry, which means that the desired coupling between the apertures and the stripline conductor is obtained.
Undesired modes are suppressed by virtue of the fact that the distance from each respective ground plane to the most closely located main surface of the stripline conductor is essentially constant within the arrangement. In other words, it is essentially free from discontinuities, which means that the arrangement is also mechanically simple. Moreover, the distances to the most closely located main surface of the stripline conductor from each respective ground plane, and thus from the aperture of each respective ground plane, are preferably essentially equal.
As the invention makes it possible to construct an arrangement with mechanical symmetry, the same dielectric material can be used on both sides of the stripline conductor. In a preferred embodiment, the dielectric material used is air, which eliminates dielectric losses.
Further advantageous embodiments are indicated in the appended subclaims.